This invention relates to a novel electrolytic cell for the manufacture of aluminum from alumina, and to the operation of such a cell. More particularly, the invention relates to an aluminum producing electrolysis cell using bath electrolyte based on sodium cryolite, wherein the problems resulting from reduced anode to cathode gap distances achieved in previous drained cathode cells are demonstrated to have been overcome by inducing a particular manner of bath flow in the ACD gap, thus facilitating alumina feeding, removal of gaseous products, and enhanced drainage of product metal. Operation of a test electrolysis cell has demonstrated the ability to provide a plentiful supply of dissolved alumina to the electrolysis zone even at very narrow anode to cathode spacings.
A commonly utilized electrolytic cell for the manufacture of aluminum is of the classic Hall-Heroult design, utilizing carbon anodes and a substantially flat carbon-lined bottom which functions as part of the cathode system. An electrolyte is used in the production of aluminum by electrolytic reduction of alumina, which electrolyte consists primarily of molten cryolite with dissolved alumina, and which may contain other materials such as fluorspar, aluminum fluoride, and other metal fluoride salts. Molten aluminum resulting from the reduction of alumina is most frequently permitted to accumulate in the bottom of the receptacle forming the electrolytic cell, as a molten metal pad or pool over the carbon-lined bottom, thus forming a liquid metal cathode. Carbon anodes extending into the receptacle, and contacting the molten electrolyte, are adjusted relative to the liquid metal cathode. Current collector bars, such as steel, are frequently embedded in the carbon-lined cell bottom, and complete the connection to the cathodic system.
While the design and sizes of Hall-Heroult electrolytic cells vary, all have a relatively low energy efficiency, ranging from about 35 to 45 percent, dependent upon cell geometry and mode of operation. Thus, while the theoretical power requirement to produce one pound of aluminum is about 2.85 kilowatt hours (KWh), in practice power usage ranges from 6 to 8.5 KWh/lb, with an industry average of about 7.5 KWh/lb. A large proportion of this discrepancy from theoretical energy consumption is the result of the voltage drop of the electrolyte between the anode and cathode. As a result, much study has gone into reduction of the anode-cathode distance (ACD). However, because the molten aluminum pad which serves as the cell cathode can become irregular and variable in thickness due to electromagnetic effects and bath circulation, past practice has required that the ACD be kept at a safe 3.5 to 6 cm to ensure relatively high current efficiencies and to prevent direct shorting between the anode and the metal pad. Such gap distances result in voltage drops from 1.4 to 2.7 volts, which is in addition to the energy required for the electrochemical reaction itself (2.1 volts, based upon enthalpy and free energy calculations). Accordingly, much effort has been directed to developing a more stable aluminum pad, so as to reduce the ACD to less than 3.5 cm, with attendant energy savings.
Refractory hard materials (RHM), such as titanium diboride, have been under study for quite some time for use as cathode surfaces in the form of tiles, but until recently, adherent RHM tiles or surface coatings have not been available. Titanium diboride is known to be conductive, as well as possessing the unique characteristic of being wetted by molten aluminum, thus permitting formation of very thin aluminum films. The use of a very thin aluminum film draining down an inclined cathode covered with an RHM surface, to replace the unstable molten aluminum pad of the prior art, has been suggested as a means to reduce the ACD, thus improving efficiency, and reducing voltage drop. However, attempts to achieve such goals in the past have failed due to the inadequacy of available RHM surfaces, and the inability to overcome the difficulty of providing a sufficient supply of dissolved alumina to the narrowed ACD (as small as 1.5 cm). Thus, problems of alumina starvation occur at minimal ACD, including excessive and persistant anode effects. Overfeeding alumina to prevent these problems has resulted in deposits of sludge (mucking), which can clog the cell and restrain its operation.
An alternative approach to reducing energy consumption has been to smelt aluminum from aluminum chloride rather than alumina. This process requires 30 to 40 per cent less electrical power to produce aluminum than conventional electrolysis. In this process, the conventional Bayer method is utilized to convert bauxite to alumina, which is converted to aluminum chloride in a chemical plant, then smelted in an electrolytic cell. In the cell, the aluminum chloride breaks down into aluminum, which is drawn off, and chlorine, which may be recycled back to the chemical plant for the production of more aluminum chloride. Such techniques utilize a flow-through reactor, having non-consumable anodes. However, the aluminum-producing cells of this process are incompatible with the Hall-Heroult type cell, and cannot be retrofitted to an existing aluminum plant. Thus, the chloride process requires the capital expense of an entirely new installation. To a greater degree, the present invention permits appreciable energy savings in a retrofitted plant, and obviates the need for completely new facilities.
It is against this background that the present invention was developed.